Digital Rights Management
In short, it's something that prevents you from accessing the game, without proving that you "own" the game. Own here being a loose term since it's more like a subscription, as you lose all access to the non-free versions of the game if you don't keep paying.
There are lots of different forms of DRM, this game uses online DRM that requires you to sign into an account to prove that you're supporting the game on Patreon, as well as confirm which tier you're supporting at.
So until someone comes along that cares to crack the game open and spoof the DRM, you can only play the free version if you don't wanna pay.